The respiratory tract, or airways, participates in the vital process of gas exchange in order to support the demand for oxygen intake and carbon dioxide elimination. Vagal autonomic nerves control smooth muscles of the tracheobronchial tree, and thus caliber of airways, as well as liberation and movement of secretions (mucus and fluid). Control is coordinated within brainstem nuclei which regulate voluntary and autonomic outflow, relying on a rich input of vagal sensory signals from the airway tissues that in turn convey conscious sensation and trigger autonomic reflexes. Vagal sensory fibers arise mostly from cell bodies within jugular and nodose ganglia, and their activity is regulated by a range of chemical substances (Can & Undem (2003) Respirology 8(3):291-301). One such substance is ATP, which sensitizes vagal afferents and serves as a convergent mechanosensory airways signal (Weigand, Ford and Undem (2012) J Physiol. 590(16):4109-20).
ATP activates purinoceptors (e.g., P2X3 and P2X2/3), which mediate many physiological and pathological roles (See, Burnstock (1993) Drug Dev. Res. 28:195-206). ATP stimulates and sensitizes sensory nerve endings resulting in intense sensations such as pain, discomfort, urgency, itch and urge and a pronounced increase in sensory nerve discharge, largely via P2X3 receptor activation on afferent nerve fibers innervating rodent and human tissues and organs, especially the hollow viscera.
Data suggest that ATP may be released from epithelial and interstitial cells of hollow organs (such as airways, bladder) as a result of distention, movement, pressure or inflammation (Burnstock (1999) J. Anatomy 194:335-342; and Ferguson et al. (1997) J. Physiol. 505:503-511). ATP thus serves a role in conveying information to sensory neurons located in epithelial and subepithelial compartments, e.g., subepithelial lamina propria (Namasivayam, et al. (1999) BJU Intl. 84:854-860; Weigand, Ford and Undem (2012) J Physiol. 590(16):4109-20).
Undem and co-workers have reported that P2X3 and P2X2/3 receptors are widely expressed and modulate function of nodose and jugular afferent fibers in mammalian airways (Weigand, Ford and Undem (2012) J Physiol. 590(16):4109-20). Additionally, in a guinea pig model of ATP or histamine potentiation of citric acid induced cough, P2X subfamily receptors were implicated although contribution of P2X3 or P2X2/3 receptors was not deduced (Kamei, Takahashi, Yoshikawa, Saitoh (2005) Eur J Pharmacol. 528(158-161); Kamei and Takahashi. (2006) Eur J Pharmacol. 547:160-164). Finally, it has been shown in human studies that patients with airway disease associated with cough and breathlessness (such as asthma, COPD or pulmonary fibrosis) have excess ATP concentrations in their airway fluids (Esther, Alexis and Picher. (2011) Subcell. Biochem. 55:75-93; Lommatzsch et al. (2010) Am J Respir Crit Care Med. 181(9):928-34), and that the inhalation by asthmatic patients of nebulized ATP is able to activate airways sensations leading to urge to cough and precipitating cough itself (Pellegrino et al. (1996) J Appl Physiol. 81(2):964-75.; Basoglu et al. (2005) Chest. 128(4):1905-9), although the site of action of this effect of ATP, and receptor(s) involved have not been elucidated.
There is accordingly a need for methods of treating diseases, conditions and disorders mediated by P2X3 and/or P2X2/3 receptors, as well as a need for compounds that act as modulators of P2X receptors, including antagonists of P2X3 and P2X2/3 receptors. Such diseases and disorders are herein shown to include cough, chronic cough and urge to cough, including cough associated with a respiratory disease or disorder. Chronic cough is distressing and functionally disabling, and no novel licensed treatments for cough have appeared in approximately 50 years. The present invention satisfies these needs as well as others.